1. Field of the Invention
The present invention relates to a CATV cable modem for receiving digital data transmitted over a cable television network, and more specifically pertains to a serial tone data transmission in the presence of unwanted interference.
2. Description of the Prior Art
It is well known that cable networks are being used for data and telephone services while continuing to supply cable TV service. Usually these systems are Hybrid Fiber Coaxial (HFC) systems. HFC systems utilize fiber optic cable to deliver the broadcast signals to a residential neighborhood from the head end. The signals are distributed throughout the neighborhood by means of coaxial cable. This arrangement is used because although fiber optic cable has a very high capacity and is virtually distortion free, it is more expensive and requires more expensive modulators and demodulators. Although coaxial cable has a lower capacity and generally introduces more distortion, it is less costly and is already in place throughout most of Europe and North America. These cable systems comprise a head end, fiber optic transmitters and receivers, fiber, cable, RF amplifiers, taps and drop cables. Early cable systems were designed for only one direction, that being from the head end to the home. Systems today are designed to handle data transmission in both directions. Typically, the cable is made bi-directional by using frequencies from 50 MHz to 550 MHz (or higher) for the downstream transmission, which is towards the home, while frequencies from 5 MHz to 42 MHz are used for the upstream transmission, which is from the home. To provide amplification in both directions, without oscillation, the amplifiers separate the upstream and downstream signals by filtering and then amplifying them separately. Earlier systems generally required upgrading for bi-directional transmission, and many systems were upgraded for upstream transmission of pay TV signals. However, the overall quality of the upstream path was not as good as it could be if the upgrade were designed for data transmission.
In handling data over cable systems, the cable network is shared with the conventional cable TV along with other traffic. When in the downstream direction, a portion of the downstream bandwidth is allocated for data traffic. The data band can be a contiguous band or it may be interspersed between the TV and other traffic. Typically, the downstream bandwidth is much greater than the upstream bandwidth. This works out well, since in most data applications, the amount of data flowing toward the end user is much greater than the amount of data flowing from the user. In a typical system, there would be many modems operating at once at different frequency bands, and each frequency band is also time shared among many modems, thereby allowing hundreds of users to be able to use the system simultaneously. In addition, the cable system is segmented into neighborhoods wherein each neighborhood will have its own fiber optic cable, so that the capacity of the system can be multiplied by the number of neighborhoods serviced.
Ingress is interference which is inadvertently coupled into the cable plant and it is a significant problem in cable systems. The upstream path is subject to more ingress that the downstream path. This is because the lower frequencies couple into the cable more easily than do the higher frequencies, and also because in the upstream direction all ingress is funneled back to the head end, whereas in the downstream direction, ingress only affects the portion of the system that is downstream from the point where the ingress enters. Ingress is more commonly found at frequencies below 15 MHz, although it can also be found at higher frequencies. Ingress is predominantly picked-up in the drop cable, which is the final connection to the house. Among the many sources of ingress are: HF radio broadcast transmissions, electromechanical devices, mobile radios, and CB radios. Ingress is usually concentrated at a few discrete frequencies.
Cable modems fall into two broad categories, parallel tone modems and serial tone modems. Parallel tone modems operate by dividing the band into many sub-bands and then sending data at a low rate through each sub-band. This technique is also known as Discrete Multi-Tone (DMT). This approach has the advantage in that, if any ingress is present in one or more sub-bands, just those sub-bands will be effected. Systems can then be designed that will detect the presence and location of ingress so that data can be sent to just the ingress-free part of the frequency band. However, since the power in each sub-band is much less than the total power, the ingress power required to interfere with a sub-band is much less. The parallel tone approach is more sensitive to ingress, but it does degrade gracefully and gives up just a small amount of data capacity to avoid the ingress. This approach is generally more expensive to implement than the serial tone approach, and does not work well in the presence of ingress that occurs at many parts of the frequency band.
Serial tone modems send one symbol at a time at a rate such that the signal occupies the entire bandwidth. There may be several modems operating at different frequencies at the same time, but each modem operates serially and independently of the others. Serial tone modems are more robust than the parallel tone modems, but once the ingress becomes large enough to interfer with the signal, all data transmission is degraded or lost. The most common type of modulation used in cable modems is Quadrature Amplitude Modulation (QAM). Usually, 16QAM or 4QAM is used for the upstream and 16QAM, 64QAM or 256QAM is used for the downstream. The cable industry has selected serial tone modems as the preferred method of providing over cable, a cable consortium called the Multimedia Cable Network Systems (MCNS) consortium has issued a specification (Data Over Cable Interface Specifications/Radio Frequency Interface Specification SP-RF1101-970326) for cable modems which will enable equipment from different manufacturers to inter-operate. This specification calls for a serial tone modem using 4QAM and 16QAM for the upstream and 16QAM, 64QAM or 256 QAM for the downstream. The MCNS specification provides for three methods of combating ingress, these are: a) changing to to a more robust signal constellation, b) reducing the symbol rate, and c) changing the transmission frequency to a frequency where there isn""t any ingress present. Changing the signal constellation, to a more robust one, reduces the data rate as does reducing the symbol rate. Changing the signal frequency to avoid ingress means that part of the band is unused, so the overall system throughout is reduced. Often it may be necessary to reduce the data rate in order to fit the signal into the available ingress-free bands. All combinations of these methods are called for in the MCNS specification. The present invention can be used with modems designed to meet the requirements of the MCNS specification as well as other serial tone modems.
The present invention provides a means of removing ingress in cable modems, without reducing the data rate or changing the frequency of the signal. Furthermore, the present invention is used at the head end only, and does not require any change in the hardware or software in residential modems. Since the number of head end modems is very small in comparison to the number of residential modems, the increase in the system cost is very low for what amounts to a substantial increase in system throughput and capacity. The present invention uses a variable band stop filter bank to remove ingress, and a non-linear equalization arrangement to compensate for the distortion introduced by the band stop filter bank. A spectrum monitor is used to detect the presence of ingress and its frequency. When ingress is detected, a band stop filter is formed. The band stop filter has a notch at the frequency of the ingress. The equalizer is set to make compensation for the distortion which is introduced by the band stop filter. The band stop filter may have a multiplicity of notches to eliminate ingress at a multiplicity of frequencies. If the band stop filter were implemented using a conventional FIR approach, the number of taps required would be excessive. Two ways of overcoming this problem are the fast convolution approach and the time mutiplexed polyphase filter approach. With todays technologies the fast convolution approach lends to the most efficient implementation. The equalizer removes the distortion introduced by the band stop filter. The equalizer consists of an inner equalizer and an outer equalizer. In some applications the inner equalizer alone may provide adequate performance. When ingress is detected by the spectrum monitor, a microprocessor or Digital Signal Process (DSP) is used to calculate the optimum equalizer settings.
Accordingly, it is a principal object of the invention to provide a means of removing ingress in cable modems without reducing the data rate or changing the frequency of the signal.
It is another object of the invention to be used only at the head end, thereby eliminating any need for hardware or software in the home.
It is a further object of the invention to provide a method of detecting the presence of ingress and removing the same.
It is an object of the invention to utilize a variable band stop filter bank to remove ingress and a non-linear equalization arrangement to compensate for the distortion introduced by the band stop filter bank.
It is an another object of this invention to remove ingress in digital radio communications.
These and other objects of the present invention will become readily apparent upon further review of the following specification and drawings.